hidden. In order to preserve this chemical change in the
silver salts the sensitive plate must be washed or soaked in a solution
which will form an opaque compound with the part of the salt which has
been acted upon by the light. As it is necessary to have a light to
watch the process and stop it when it has been carried far enough, we
must have a light free from actinic or blue rays. We therefore darken
the room and use a red light, for the red rays have little or no actinic
power.

As we watch the chemical change which takes place in the sensitive plate
when covered with what we call the developer, we notice black patches
appear here and there on the plate. These are the places which have been
exposed to the strongest actinic rays. All bodies radiate or reflect
light, some more than others. A piece of yellow silk may appear to the
eye lighter in color than a piece of blue silk, but when the two pieces
are photographed it will be found that the yellow photographs much
darker than the blue silk. This is because the yellow silk does not
reflect actinic rays, while the blue does, and therefore the sensitive
plate is more strongly affected by the light reflected from the blue
than from the yellow silk. The yellow-colored silk possesses the
illuminating power which causes it to make a room look bright and sunny,
while the blue silk possesses the chemical power which affects the
sensitive salts.

If the sensitive plate has been exposed to a landscape, the strongest
actinic rays come from the blue sky, and as the chemical used to develop
or bring out the image affects the part which has been exposed to the
strongest light most quickly, the result in the negative is the opaque
deposit which covers all that part of the plate exposed to the light
from the sky. After the image has appeared on the plate it must undergo
another process to make it permanent. It must be placed in a chemical
solution which shall dissolve the silver salts from the parts unaffected
by the actinic rays. Where the actinic light has been the strongest the
glass will be covered with a black deposit, and where the light has not
reached the plate with sufficient force the salts will be dissolved and
the glass will be clear, while the high lights, the shadows, and the
half-tones will show just how much each object reflected actinic rays.

We manage our cameras, but the sun is the real work-man. What he does is
well worth learning, for it enables us to tell b